feat(simulation): T-1170 A2/A3 + T-1168 A5 — course inventor, coast termination, riparian signal

A2 (river_course.rs): Stage A rung-independent valley-seeking control
path (chord/8 stations, k=5 bilinear-scored candidates + continuity
penalty); Stage B rung-indexed perpendicular warp on GLOBAL arc-length
(window-independence, Ruling 1e), band chord/2 down to min_wavelength_m
hard-truncate, sine taper to zero at anchors, amplitude min(8% chord,
half-cell) slope/class-scaled. SeedDomain::RiverCourse=17, distinct
salt. Wire: RiverCourse{edge_id,class,points,terminus} on
DistrictWindowLayer.courses (serde-default); bbox-culled, window-
cropped +1 station. TerrainAnalysisCache retains Layer1Output (the
gen_queue:626 discard, Ruling 4b). A3: mouth termination walks
stations sampling the window's OWN rung-consistent morphology verdict,
6-iteration bisect; land-at-anchor probes one segment then None;
EdgeDrain never probes. A5: near_perennial_water point-to-segment
predicate (D-239 §8 governed bands 1-3m class-scaled) threaded through
both batch and window paths; Region always false; never touches
moisture_q. Discipline closed: dormant zoom-ladder bench run + numbers
recorded in the design doc (District 3.009/1.785, Quarter 1.823
us/cell, Region window 0.617ms); course-cost bench CAUGHT a real
+12-36% per-cell riparian scan regression -> precomputed bbox O(1)
reject (60ns->2.2ns/call), final delta +3.4-6.9% at budget; three
determinism tests (overlapping-window byte-identity, cross-rung
amplitude bound, warp-stream cross-correlation r<0.3); goldens: window
sweep gained a verified course-bearing position (pure append), new
river_course golden at both rungs, believability verified unchanged.
Revert-verification discovered the pole-row branch is structurally
unreachable (flow_direction bounds-check) — the real edge-drain path
is k<0 flat-plateau; test fixture rewritten to exercise reality.
scale.rs stale comment fixed. Full cargo test green.

Tickets: T-1170, T-1168

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/src/seed.rs

@@ -146,6 +146,15 @@ pub enum SeedDomain {
     /// independent entropy — the `assign_block_tags` lesson (distinct
     /// sub-chains per field, not one shared roll) applies here too.
     TraitExterior = 16,
+    /// River course invention (T-1170, Ruling 3a): the linear sibling of the
+    /// coastline warp, keyed by `edge_id` (the packed upstream-cell u32 of a
+    /// D8 river edge, see `atlas::river_course`). Distinct domain so the
+    /// course's Stage-B perpendicular warp octaves can never correlate with
+    /// the coast warp, terrain scatter, or vegetation massif fields sampled
+    /// at the same world position (D-224 domain separation, the module's own
+    /// `RIVER_COURSE_WARP_SALT` provides a second, position-keyed layer of
+    /// isolation on top of this domain tag).
+    RiverCourse = 17,
 }
 
 /// A position in the deterministic seed tree (D-224).
@@ -311,6 +320,7 @@ mod tests {
         assert_eq!(SeedDomain::TraitDistrict as u64, 14);
         assert_eq!(SeedDomain::TraitSwerve as u64, 15);
         assert_eq!(SeedDomain::TraitExterior as u64, 16);
+        assert_eq!(SeedDomain::RiverCourse as u64, 17);
     }
 
     #[test]